Believe it or not I had really good results with Banggood on one occasion where it was easily apparent that the language barrier was the hold up in the works. I ended up typing out a message to them and used Google Translate to convert it to chinese. I then copy and pasted that into my message. They of course sent me a message back in Chinese and I was able to use Google translate again to render it in English. Problem was solved fairly quickly that way. It was a very simple problem like you are having but the bottle neck was the language.
Oh and by the way, try to keep it super simple and to the point. They don’t understand some of our slangs and sentence structure.